TROPICS Vol. 2 (1): 23 - 28 Issued August, 1992

Population Structure of the Dominant Palm in the Understory of a Mexican Lowland Rain Forest

Ken OYAMA Centro de Ecologia, U.N.A.M. Apartado Postal 70-275, Mexico, 04510, D.E Current address: Department of Biology, University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153, Japan Rodolfo DIRZO Centro de Ecologia, U.N. A.M. Apartado Postal 70-275, Mexico, 04510, D.E Guillermo IBARRA-MANRIQUEZ Estaci6n de Biologia Tropical Los Tuxtlas, Apartado Postal 94, Los Tuxtlas, Veracruz, Mexico

Abstract Population structure was analyzed for five of the most common species of palms, Astrocaryum mexicanum, Bactris tricophylla, oblongata, C. tepejilote and Geonoma oxycarpa in a Mexican lowland rain forest. Height and cover of palms with a height> 1.5 m were measured in three 600 m2 plots. The most common species in terms of density were C. tepejilote and A. mexicanum. The highest accumulated cover corresponded to A. mexicanum.Taller palms of C. tepejilote showed higher cover. Almost all palm species had the same architectural model (Comer) and B. tricophylla had the Tomlinson model. Richness of palm species was low compared to other tropical forests although the understory palms in this community represent more than 50 % of all the individuals of the understory vegetation at Los Tuxtlas. The Chamaedorea was the most diversified (5 species) while the other genera had only one species.

Key words: Mexico / Neotropics / palms / population structure / species diversity / rain forest

Few studies have been carried out on the diversity and structure of the palm components within tropical forest communities, although palms usually occur in high numbers in mixed tropical forests. Palm species are very diverse in some lowland rain forests like in Sunda Shelf and New Guinea and of Central and South America (Uhl & Dransfield,1987). Palms may also occur in large stands of single species and dominate the vegetation. For example, Nypafruticans in Borneo and Sumatra (Uhl & Dransfiled, 1987), Raphia taedigera in Costa Rica and Caribbean plains (Myers et aI., 1985), Copernicia alba in South America (Uhl & Dransfiled, 1987) are some of the most striking examples. However, the population structure and patterns of distribution have been rarely studied and poorly quantified (Pinero et al., 1977; Kahn & de Castro, 1985). A study of the palm flora of Los Tuxtlas natural reserve in Southeast Mexico describes 10 species (Ibarra-Manriquez, 1988). Biogeographically, this area is very important because it represents the northernmost limit of lowland rain forest in the Neotropics (Dirzo & Miranda, 1991). Community structure studies at Los Tuxtlas have shown that palms are a very impor­ tant component of the vegetation (Pinero et al., 1977; Bongers et al., 1988) and in some patches of the forest, palms constituted more than 50 % of all understory trees with a girth breast height of > 3.3 cm (Oyama, 1984; Oyama et aI., in prep.). In this report, population K. OvnMA, R. DRZo & G. ISIRRA-MANRTQuEz structtue and relationships between cover with height are presented for the five most common palm species of I-os Tuxtlas.

MATERIALS AND METHODS

This study was conducted at the Estaci6n de Biologfa Tropical Los Tuxtlas in Southest Mexico. The vegetation of this 700 ha r€serve is classified as lowland rain forest (Miranda & Hern6ndez-X., 1963). Detailed description of this area can be found in lot-Helgueras (1976) and Ibarra-Manr(quez and Sinaca-Colfn ( 1 987). Palms with a height > 1.5 m werc tagged and mapped in three plots of 600 m2 (30 x 20 m). Height and cover were recorded for all palms. Height was measured directly. Cover was estimated from the largest crown diameter @r) and the diameter perpendicular to D1 accord- ing to the formula of a circle. The population structue of the species was assessed using 0.50 m height-classes in order to comparc all species. Differences in cover among height-classes were assessed with a non- parametric analysis of variance (Kruskal-Wallis-test) (Zar, 1974) for each species. Architectural models of palms were characterized following the models proposed by Hall6 et al. (1978) basdd on a set of morphorogical characters that includes the life-span of meris- tems and the degree and type of differentiation of vegetative meristems.

RESULTS

Density, cover, architecture model and maximum height for the five species of palms are presented in Table l. (0.1 / mz) had the highest density followed by Astrocaryum mexicanurn (0.06 / m2),C.oblongata (O.02 | m2), Bactris tricoplrylla (0.01 / mz) andGeonom4 oxycarpa (0.001 / m2). Astrocarywn mexicanwnhadthe highest cover followed by C. tepejilote, B . tricoplrylla, C . oblongata and G. orycarpa (Table 1). All the palms were located in the understory layer of the vegetation (below 8 m) with some of A. mexicanwnreaching the maximum height (7.6 m). The population structure of A.mexicanum,C.tepejilote andC. oblongata wascharacterizedbyahighnumberof plants in the smallest categories of height followed by a continuous decrement in numbers in

Table 1. Architecture model, density, cover, basal area and maximum height of the five most common species of palms at Los Tuxtlas. Species Model Density Cover Height (#/ 1800 ft2) (m2) (m) ,4strocaryum mexicanum Corner 115 1244.25 7.6 Chamaedorea tepejilote Corner 173 331.73 5.8 Chamaedorea oblongata Corner 34 28.3 5.0 Bactris tricophylla Tomlinson 18 62.34 5.0 Geonoma orycarw Corner 2 6.32 2.0 Population sEucture of the dominant palm in a Mexican lowland forest 25

Astrocarytan mcxlcanum Asttocarturn nexlcanum

>20q) tr G| c, E a \/ 10 q)It li C) ri lo (Jso

lA Gt u? Fi rA ? rA ra rA U? U? l.| ra |a r.. .taaaaa €' F $ vl d rrt ! ln ra rc ra F F F|GlF)?lrt\eF H|lltttvra\cFItlta.r 60 Chamaedorca tcpejilote Chamacdorea lepejilote

50

>.40 CJ N (utr E (t=30 L o, || f. 20 c lx c)

IO

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Chamacdorcd oblongata Chamaedorea oblongata

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Baclris tricophylla Bactris tricophylla

Gr rt ! ra vl Yl F tl G C| v: F) vl t? ur \o vl l\ o FlGlttlirO\OF": "l "l "l "l ": ? "l "! -Glarr!tut\ots Height (m) Height (rn)

Fig. 1. Population structure and relationships between height and cover for four palm species at Los Titxtlas forest. Means * S,E. are presented for cover. Geonoma orycarpa data were excluded due to low sample size. K. OyAMA, R. DIRzo & G. InlnRl-MauruQvgz higher height-classes (Fig. 1). The population structure of B. tricoplrylla and G. orycarpa were not well represented due to the low number of plants recorded in the sampling plots. Cover varied significantly among height-classes only in C. tepejilote (H=16.7; P < 0.05) with a tendency for cover to be large in the largest heigh-classes (Fig. 1).

DISCUSSION

Few studies exist on the structure of palm vegetation in nopical communities. Population structure of some species and diversity of the palm flora has been found to vary geographical- ly (Uhl & Dransfiled, 1987) and locally in relation to environmental factors (Kahn & de Castro, 1985). Thirty-two species of palms were differentiated under different hydrcmorphic conditions with few species dominating different zones in Central Amazonia (Kahn & de Castro, 1985). l,os Tuxtlas forest is not so diverse in its palm flora although the whole number of individuals is comparable to that of other ropical communities. We found 342 individuals (> 1.5 m) in 1800 m2 (or 1900/ha). If we include the smaller individuals of all palms species at los Tuxtlas, the density will be, at least double or more. For C. tepejilote only, more than 750 plants (including juveniles and immature palms) have been recorded in the same area (Oyama,1987; Oyama, 1990; Oyama et al.,in prep.). Chanwedorea tepejilote and A. mexicanum werc the most common species followed by C. oblongatct, B. tricophylla and G. orycarpa. Only two individuals of G. orycarpa were found in the 1800 m2 sampled. Although the presence of many rare species coexisting with a few dominant species is a very common feature of tropical communities (Clark, 1986; Whitmore, 1988), there is no clear explanation to this phenomena. Hypotheses relating genetic drift (Fedorov, 1966), habi- tat specialization (Ashton, 1969), biological interactions (Janzen, 1970; Connell, 1971) and the pattem of natural regeneration of forest @enslow 1987) have been proposed. Ten palm species have been described for Los Tuxtlas (Ibarra-Manr(quez, 1988) but only five were included in the present study because we only measured palms higher than 1.5 m. Reinhardtia gracilis and Clnmaedorea ernesti-augustii arc two other very common but small palm species at l,os Tuxtlas (A. Mendozaand S. Bullock, pers. coms.). Palm species differed in several aspects. For example, A. mexicarutrn, B. tricoplrylla and G. orycarpa are monoe- cious while Clamaedore4 spp. are dioecious. Flowering time is also differcnt among species; A. mexicaru*n, B. tricophylla andC. oblongata in the dry-season (March-May) but C. tepejilote and G. orycarpa from September or later (Ibarra-Manr(quez, 1988). The great differentiation of some genera and the monospecificity of others is still one of the enigmas in tnopical communities. The genus Clnnaedorea is represented by five species at I.os Tuxtlas but other genera had only one species. There was not any apparcnt correlation to an environmental or biological factor for Chamnedorea species. Kahn and De Castro (1985) reported ecological differentiation of species by occupying different soil conditions, and in a well-drained soils they found 11 species of Bactris and six species of Geonoma in 7200 m2 plot. In a more extensive study, Dransfield (1984) found 1ll species belonging to 20 genera in 52864 ha in Sarawak although palms did not dominate the vegetation within this locality. Geographically, a great diversity of palms may be found in very different regions. For example, the Choc6 region of Colombia and parts of the island of Borneo may be singled out as being extraordinarily rich in palms (Uhl & Dransfield,l9ST). hs Tuxtlas forest, being a locality close to the northern limit of the distribution of the tropical rain forest in Population structure of the dominant palm in a Mexican lowland forest 27

Continental America (Dirzo & Miranda, 1991) may represent a marginal region of palm distribution in which low diversity is found due to historical factors and speciational process­ es, but with a high density of few species by the occupancy and expansion of local ecological niches.

ACKNOWLEDGMENTS Thanks to S. Torano and M. Nishizawa for the translation of the summary to Japanese and S. Sinaca-Colin for field assistance.

REFERENCES

Ashton. P. S. 1969. Speciation among tropical forest trees: some deductions in the light of recent evidence. Biological Journal of Linnean Society 1: 155-196. Bongers, E, Popma, J., Meave del Castillo, J. & Carabias, J .. 1988. Structure and floristic composition of the lowland rain forest of Los Tuxtlas, Mexico. Vegetatio 74: 55-80. Clark, D. A. 1986. Regeneration of canopy trees in tropical wet forests. TREE 1: 150-154. Connell, J. H. 1971. On the role of natural enemies in preventing competitive exclusion in marine animals and rainforest trees. In: P. J. den Boer & Gradwell, G. (eds.), Dynamics of Populations, 298-312. PUDOC. Denslow, J. 1987. Tropical rainforest gaps and tree species diversity. Annual Review of Ecology and Systematics 18: 431-451. Dirzo, R. & Miranda, A. 1991. Ellfmite boreal de la selva tropical humeda en el Continente Americano. Contracci6n de la vegetaci6n y soluci6n de una controversia. Interciencia 16: 240-247. Dransfield, 1. 1984. The palm flora of Gunung Mulu National Park. In: A. C. Jenny (ed.), Studies on the Flora of Gunung Mulu National Park, Sarawak, 41-75. Forest Department, Kuching, Sarawak. Fedorov, A. A. 1966. The structure of the tropical rain forest and speciation in the humid tropics. Journal of Ecology 54: 1-11. Halle, E H., Oldeman, R. A. & Tomlinson, P. B. 1978. Tropical Trees and Forests: an Architectural Analysis. Springer-Verlag, New York. Ibarra-Manriquez, G. 1988. The palms of tropical rain forest in Veracruz, Mexico. Principes 32: 135-139. --, G. & Sinaca-Colfn, S. 1987. Listados floristicos de Mexico. VII. Estaci6n de Biolgia Tropical Los Tuxtlas, Veracruz, Mexico. Janzen, D. H. 1970. Herbivores and the number of trees species in tropical forests. American Naturalist 104: 501-528. Kahn, E & de Castro, A. 1985. The palm community in a forest of Central Amazonia, Brazil. Biotropica 17: 210-216. Lot-Helgueras, A. 1976. La Estaci6n de Biologia Tropical Los Tuxtlas: pasado, presente y futuro. In: A. G6mez-Pompa, del Amo, S., Vazquez-Yanes, C. & Butanda, A. (eds.), Investigaciones Sobre la Regeneraci6n de Selvas altas en Veracruz, Mexico. I, 31-69. Compania Editorial Cientifica, S.A., Mexico. Miranda, E & Hernandez-X, E. 1963. Los tipos de vegetaci6n de Mexico y su clasificaci6n. Bol. Soc. Bot. Mexico 28:29-178. Myers, R. L., Duvall, M. & Kiester, R. 1985. The ecology of Raphia taedigera in Costa Rican palm swamps. Abstracts, Associationfor Tropical Biology Meetings 1985: 39. 28

Pinero, D., Sarukhan, J. & Gonzalez, E. 1977. Estudios demograficos en plantas. Astrocaryum mexicanum Liebm. I. Estructura de las poblaciones. Bol. Soc. Bot. Mexico 37: 69-118. Oyama, K. 1984. Biologfa comparativa entre individuos masculinos y femeninos de Chamaedorea tepejilote (Palmae). Tesis Profesional, Facultad de Ciencias, U N.A.M., Mexico. Oyama, K. 1987. Demografia y dinamica poblacional de Chamaedorea tepejilote Liebm. (Palmae) en la selva de Los Tuxtlas, Veracruz (Mexico). Tesis de Maestrfa, Facultad de Ciencias, U.N.A.M., Mexico. --, 1990. Variation in growth and reproduction in the Neotropical dioecious palm Chamaedorea tepejilote. Journal of Ecology 78: 648-663. Uhl, N. W. & Dransfiled, J. 1987. Genera Palmarum. Allen Press, Lawrence, Kansas. Whitmore, T. C. 1988. The influence of tree population dynamics on forest species composi­ tion. In: Davy, A. J., Hutchings, M. J. & Watkinson A. R. (eds.). Plant Population Ecology, 271-291. Blackwell Scientific Pub., Oxford. Zar, J. H. 1974. Biostatistical Analysis. Prentice Hall, Englewood Cliffs, NJ.

Received March 27,1992 Accepted July 12, 1992

K. OYAMA, R. DIRZO, G. IBARRA-MANRIQUEZ 7J t' :/ ~ 0).I~Im4i§

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1WI1: 01t' -C7~tlJE ~ tLt-:o WltO)7i11iIt'llli C. tepejilote t A. mexicanum -c'~ "J t-: o tJ /'{-mHli.lf l1i bit 1t'0) Ii A. mexicanum -c'"J6 "J t-:o Chamaedorea tepejilote i.lfiji b WJ3to)f.lj It , -\' ~-c'"J6 "J t-: i.lf, tJ /'{- OO~l b *~ i.I' "J t-:o Ii t Iv t'T""' -C o)-\' ~O)lIO) 7 -:f-T :r.. 7 T l' - Ii Corner .:c T )v ~/F LJ-:i.lf, B. tricophylla Ii Tomlinson .:c? )v-c'"J6 "J t-:o Los Tuxtlas O)*F"J~~O), 1WI1*0) 50% £J.-.ti.lf-\, ~jijf-c'6 ~ ty tL -C It, J.J i.lf, -\' ~0)~ljijf~Ii1tHO)~'1ifMI:1tiXT J.J t T <~ It'o Chamaedorea JJ,IHl b~ < -C 5ll~ "J t-:i.lf, 1tHO)JJ,Ii-r-tL-t"tL nlli.l'~i.I' "J t-:o